duplicate an json_object

[chennqqi]

I add an function json_object_duplicate to duplicate an json_object. Do you accept this PR?

[hawicz]

First of all, this doesn't compile due to the mismatched function prototype, but even if you fix that the approach of serializing the object tree into a string and then parsing it back out, while obviously convenient, is not one that I will accept into the library.
Instead, the code should walk the tree and copy each object directly. The recently added json_c_visit() function may be a useful way to do this.